Brent North Sea crude for delivery in October added 60 cents to $113.25 a barrel just after midday.
New York's main contract, light sweet crude for October increased by 33 cents to $94.95 per barrel.
Volumes were thin and traders were "very cautious, very defensive" ahead of Bernanke's closely-watched speech later Friday at Jackson Hole, Wyoming, said market strategist for IG Markets Singapore Justin Harper.
"I just think we're caught in the crosshairs of Jackson Hole where people are being very cautious across the board," he told AFP.
"Volume is so thin you can change the market very quickly," he added.
Investors are watching if Bernanke would indicate a fresh stimulus package for the US economy.
IG Markets Singapore said in a commentary however it was unlikely that the US central bank chief would announce a new round of quantitative easing, known as QE3, because of signs the US economy was stablising.
"On the eve of the long-waited, highly-anticipated and much-hyped Jackson Hole symposium the watchword was defence as traders reduced their risks," it said.
"High hopes of QE3 have been edging down this week as US economic data continues to show mild strengthening and less need for central bank intervention," it added.
This week, oil market sentiment was dictated by hopes of central bank stimulus measures, speculation over the possible release of strategic crude stockpiles, mixed data and hurricane-linked US supply concerns.
"The markets have been torn this week by rumours of a stockpile release and mixed signals from the economy after US GDP was revised up," said analyst Tom Pering at British-based energy consultancy Inenco.
"Despite this, oil is ready for a fall and unless the Federal Reserve or the European Central Bank indicates any positive policy measures, this trend is likely to continue next week.
"Many investors feel oil is over-priced and are looking for falls. Meanwhile, although the disruption caused by Hurricane Isaac was minimal, this has still kept a dampener on prices."
Fears about disruption to supply in the US Gulf from Hurricane Isaac had lifted prices but, in the end, those concerns proved unfounded.
The Gulf of Mexico is the hub of US offshore energy production, accounting for 23 percent of crude oil output and 7.0 percent for natural gas.
The Gulf coast's facilities also have more than 40 percent of total US petroleum refining capacity and 30 percent of natural gas processing plant capacity.
